This invention is directed to a system and method for the injection molding of high quality molded articles of various shapes, and more particularly, to a system and method for the molding of molded articles which have substantially homogenous molecular orientation such that they do not show unidirectional molecular orientation or weld lines, and more particularly, to such a molded article having an accurately located central hole. More particularly, the system is in the form of a hot runner valve gate nozzle which performs multiple tasks on molten resin to achieve a homogeneous molecular orientation of the molten resin.
As is known in the art, during injection molding of articles such as PET preforms, unidirectional molecular orientation and weld lines are difficult to avoid using previously developed hot runner valve gated nozzle designs. This defect of these designs presents a major source of concern as such weld lines and unidirectional orientation are potential causes for the weakness of a blown PET bottle that may subsequently be filled with a carbonated beverage under pressure, thereby increasing the risk of a container rupture. In addition, weld lines may also reduce the strength or have an adverse effect on the dimensional accuracy of injection molded precision parts such as gears, which have a central hole, especially those used for high load applications. Also, for articles used in optical applications, such as the compact disc (CD) and digital video disc (DVD), weld lines and unidirectional molecular orientation induce birefringence, which is an unacceptable defect given the fact that information carried by pits embedded in the surfaces of these discs has to be retrieved by passing a laser beam through the plastic substrate from which they are formed, from a side Located opposite the pits. Injection molding of CDS and DVDs is even more challenging, considering that both of these molded articles must have a precisely located central hole. Ideally, CDS and DVDs should be molded using a sprueless injection process in order to save plastic resin, reduce the cycle time and simplify the design of the mold and the injection molding machine. The former developments in the art, however, fail to teach such a system and more particularly, fail to teach a hot runner valve gate nozzle and mold, and method which are capable of spruelessly producing CD's and DVD's with an acceptable birefringence level and within tight geometrical tolerances.
Prior developments in the molding art discuss various method for producing plastic articles which do not present weld lines or unidirectional molecular orientation. However, these method do not provide satisfactory results with regard to using hot runner manifolds and nozzles.
Some of the more pertinent developments in the art are discussed as follows.
With the intent to reduce the contact between the movable stem and molten resin so as to prevent the appearance of weld lines caused by splitting of the molten resin around a valve stem, U.S. Pat. No. 4,412,807 to York discloses a valve stem located parallel to the flow of the molten resin. The valve stem therein has very limited contact with the resin which is very close to the nozzle tip. While weld lines are reduced using this design, a substantial core shift is introduced because the flow of the resin is not symmetrical with respect to the cavity gate and there is a tendency for the resin to preferentially fill one side of the cavity faster then the other.
Similar to York, U.S. Pat. No. 4,925,384 to Manner discloses a hot runner valve gate which also includes a valve stem positioned parallel to the flow of the molten resin. However, this stem is only in partial contact with the molten resin along the inner melt channel of the hot runner nozzle. The melt does not fully surround the stem, thus giving a slight improvement with respect to the appearance of weld lines. However, since the melt channel is again laterally shifted with respect to the cavity gate, some weld lines and core shift still appear. In addition, guiding the stem only by the upper portion thereof represents another drawback of this system.
Further, U.S. Pat. No. 5,104,307 to Brink teaches a valve gate stem that makes an angle with respect to the flow of the molten resin and acts upon a ball shaped closing pin. By using this design, there is no contact between the stem and the resin. However, this design clearly increases the size of the hot runner nozzle, is very difficult to seal with respect to leakage around the ball shaped pin, and cannot be used in many applications such as in cases where molded articles are to be formed having a precisely located central hole.
Another approach to improve the quality of plastic articles and to prevent the formation of weld lines has been to use non-valve gated hot runner nozzles, as disclosed in U.S. Pat. No. 4,965,028 to Maus et al. An extended reference list to patents directed to this type of approach is also discussed in this patent. Maus et al. discloses and claims a melt conditioning elements comprising heating, filtering and mixing features located just upstream of the molded gate. These features are proposed to improve the quality of the melt and accordingly, the quality of the plastic article to be molded. This mixing feature as disclosed and Maus et al. is not entirely new to that disclosure and reference is accordingly made to German Patent Application DE3201710 A1 of Gellert.
Gellert teaches a generic twisted strip located in the nozzle which is used to produce turbulence and to reduce uniaxial properties in the molded articles. The mixing element of Gellert is not very effective since it does not split and remix the molten resin flow aggressively enough to prevent the formation of weld lines, which are caused by the splitting and twisting of the incoming stream of molten resin into multiple streams that reach the cavity. The melt conditioning element of Maus et al. is, however, more effective as a mixer but causes a relatively high and non-desirable pressure drop that increases the cycle time of the injection molding machine. For both of the hot runners disclosed in Maus et al. and Gellert, valve gate stems are not used and accordingly, these devices have a substantially limited range of application.
Still another approach for avoiding the unidirectional molecular orientation of melt is shown in several patents which teach different manners for inducing helical movement to the molten resin around the stem of a hot runner valve gate. Accordingly, reference is made to U.S. Pat. No. 4,303,382 of Gellert that teaches three helical channels located in the inner wall of the nozzle that surrounds a valve stem. These channels end up just prior to the tip of the nozzle and are used to impose a curving motion to the melt when it enters the mold cavity. While this design will avoid unidirectional molecular orientation of the molten resin, it will not prevent the formation of weld lines since the three streams of molten resin are not mixed together before entering the cavity.
Other molding applications require the inducement of circular or helical movement of the molten resin where the molten resin must have a very limited amount of contact with the stem of the valve gate. Such applications include the simultaneous coinjection of several molten resins to a common gate. For example, reference is made to U.S. Pat. No. 4,512,730 of Kudert and U.S. Pat. No. 5,143,733 of Von Buren. Kudert teaches a complex hot runner nozzle design concept wherein the annular flow profile of each molten resin that arrives in the hot runner nozzle is transformed into a circular or helical profile that is totally separated from the valve gate stem. By inducing concentric movements of each individual resin for their simultaneous injection, the combined stream entering the cavity mold would fully surround each nozzle and accordingly, the layers of the streams fill the entire cavity. Von Buren teaches a different valve gate hot runner nozzle for coinjecting three resins through a single mold gate, wherein only one of the resins follows a quasi circular flow. With reference to FIG. 3, channel (104) has a helical profile that allows one thin stream of resin to fully surround nozzle body (83) for causing the same to reach the gate as a complete symmetrical molten resin stream. Neither Kudert nor Von Buren teach a valve gate hot runner that is capable of effectively mixing and homogenizing a single stream of molten resin so as to prevent the formation of weld lines in the final molded product.
With reference to U.S. Pat. No. 4,340,353 to Mayer, U.S. Pat. No. 5,324,190 to Frei and U.S. Pat. No. 5,460,763 to Asai, several sprueless injection molding methods for manufacturing compact discs having an accurate central hole are disclosed. However, no provision is made to avoid the appearance of weld lines or to avoid unidirectional molecular orientation of the melt. Specifically, Mayer teaches a plurality of radially outwardly and angularly spaced extending arms (76) and (78) used to guide a valve stem (74) along two sections. These arms, together with flow opening (89) represent obstructions of the incoming flow of molten resin, toward the mold cavity, which generate several melt lines that are visible using polarized light. Less visible weld lines are expected to be generated using the design taught in Frei and Asai since they do not have these kind of guiding arms disturbing the flow of the resin. However, neither patent teaches any means to recombine the individual streams of molten resin after they are split and prior to entering the cavity gate, so as to provide a viable solution for avoiding the appearance of weld lines. All three of these patents teach different methods and means to form a central hole in a time efficient and simple manner without discussing the high potential in each for injecting compact discs that have weld lines and unidirectional molecular orientation in the solidified resin.
The difficulty which arrives in removing weld lines caused by the interruption or splitting of the flow of plastic material is emphasized in U.S. Pat. No. 4,942,010. In this patent, a simple but very limited mixing solution for reducing weld lines is disclosed. Further disclosure which indicates the difficulty in eliminating weld lines is found in U.S. Pat. No. 4,584,154 discussing that polycarbonate, a material frequently used in molding CDS, is very sensitive to the formation of weld lines caused by the first splitting and then reuniting of separate streams.
There exists a need, therefore, for an apparatus and method for use in sprueless injection molding operations for molding high quality molded parts, and particularly, high quality molded parts having a precisely located central hole, wherein the molded part is substantially free of solidified resin having unidirectional molecular orientation and which is also free of weld lines.